Oxalic acid and esters are important compounds having a known and wide application field of considerable commercial interest. Oxalic acid may be used in the textile industry as the auxiliary agent for "stripping" in the dyeing of wool, as a bleaching agent for natural fibers, as a pickling agent for metal surfaces, especially of copper; in the industry it can be utilized as a dehydrogenating agent in condensation reactions, etc. Finally, the esters are known solvents, such as, for instance, diethylester for cellulose.
It is known how to prepare oxalic esters by the oxidative reaction of carbon monoxide and monobasic alcohols with oxygen and also quinones, preferably in a substantially anhydrous medium due to the presence of dehydrating matters, and catalyzed by redox systems usually consisting of a finely subdivided metal or of soluble salts or complexes (citrates, kelates) of a noble metal of the Pt group, such as Pd, U, Os, or of a salt and/or a complex of another metal more electropositive than the preceding ones, such as Fe, Co, Ni, Cu, Mn, etc., such as chlorides, acetates, etc., having several oxidative states. Reaction is preferably conducted in the presence of a co-catalysts and/or complexing agents consisting of soluble salts of alkaline metals (LiCl, KCl, etc.). Processes of the aforedescribed type are disclosed in U.S. Pat. No. 3,393,136 and German Patent Publication No. 2,213,435.
Processes of the aforedescribed type, however, due to the contemporaneous occurrance of secondary reactions leading to the forming of carbonates, CO.sub.2, esters (acetates, formates, etc.), cannot be considered as thoroughly satisfactory from a commercial viewpoint owing to the low yields and the relative expensive separation, purification, etc. operations.
On the other hand, other processes known in the art for producing oxalic esters such as, for example, those based on the dehydrogenation of sodium formate successively transformed into calcium oxalate acidified etc., or an oxidation of propylene with NHO.sub.3 catalyzed by Fe, Cr, etc., do not yield better results, though involving considerable technological and operative difficulties that render them less profitable especially for commercial scale productions.
Thus, it is an object of the present invention to provide a simple and economic process for preparing oxalic acid and alkyl esters of same, free from the drawbacks of the prior art and, in particular, capable of providing high yields and purity of the products and representing an actual progress with respect to the prior art.